Such a dangerous area is found, for example, in an automobile assembly factory in which a flat structure of panels or safety wire nets is held horizontally on a level between a ground and a horizontal arrangement of elevated rails for conveying car bodies or the like suspended therefrom, with the purpose of preventing the suspended objects from falling down to the ground, and for providing a standing support for workers engaged in checking and maintenance of the rails. More particularly, in the flat structure of panels or safety wire nets, there is formed an opening for lowering the suspended objects halfway with an upright lifter. The lifter and a special vertical rail section from the horizontal rails are arranged so as to pass through the opening which is formed in the panels or the safety wire nets. However, this opening is very dangerous to the workers because there is a likelihood that a worker may step into and fall into the opening carelessly and hurt oneself seriously.
Thus, a preventive countermeasure has been hitherto taken against the dangerous area. That is, a floor mat with a build-in pressure sensitive sensor is placed down adjacent dangerous areas such as the above-mentioned opening so that, when the worker steps on the mat, the sensor generates a detection signal to activate a buzzer which is placed in a proper position remote from the mat. In this manner, the sound of the buzzer causes the stepping worker to notice the immediate dangerous area. However, the known floor mat with the built-in sensor needs other electrical equipment such as an electric power source, electric wires and a control unit, so that a problem lies in the complexity of installation and the increased costs of production and maintenance.